1. Field of the Invention
The present invention relates to an optical amplifier and an optically amplifying method, and in particular, to those that are applicable for an optical amplification for use with an optical communication system and that have a bi-directional excitation light source.
2. Description of the Prior Art
Conventionally, an optical amplifier and an optically amplifing method are applied to for example an optical amplification for use with an optical communication system. Recently, as an explosive increase of the users of the Internet, it is necessary to drastically increase the transmission capacity of backbone transmission systems. As a means for increasing the transmission capacity effectively using the existing infrastructure, WDM (Wavelength Division Multiplexing), in particular, DWDM (Dense Wavelength Division Multiplexing) is becoming attractive. As a means for totally amplifying an optical signal that has been wavelength multiplexed, an optical amplifier and an optically amplifying method are becoming important.
FIG. 1 shows an example of the structure of a conventional optical amplifier. A part of an optical signal 901 that is input from a transmission path is branched to a photoelectrically converting device 933 by a directional coupler 921. The photoelectrically converting device 933 converts an output signal of the directional coupler 921 into an electric signal corresponding to the input power. A part of an output signal of an optically amplifying medium 920 is branched to a photoelectrically converting device 934 by a directional coupler 924. The photoelectrically converting device 934 converts an output signal of the directional coupler 924 into an electric signal corresponding to an output power to the transmission path.
A controlling circuit 950 controls the power of excitation light of a forward excitation light source 931 or a backward excitation light source 932 so that the output power to the transmission path becomes a predetermined power.
There are two types of excitations of an optically amplifying medium. One is a forward excitation in which the optically amplifying medium is excited from an input end and the other is a backward excitation in which the optically amplifying medium is excited from an output end. The forward excitation has an advantage of a high S/N ratio, whereas the backward excitation has an advantage of a high gain. In the conventional C band (1530 nm to 1565 nm), regardless of the forward excitation and the backward excitation, a desired gain can be obtained. However, as the number of wavelengths to be multiplexed increases, the L band (1570 nm to 1610 nm) has been used.
However, according to the prior art, since the L band requires a higher gain than the C band, when a desired gain is obtained with the backward excitation, a sufficient S/N ratio is not obtained. In contrast, when a desired S/N ratio is obtained with the forward excitation, a sufficient gain is not obtained.
Therefore, an object of the present invention is to provide an optical amplifier and an optically amplifying method that allows a higher S/N ratio and a high gain to be obtained.
According to a first aspect of the present invention, there is provided an optical amplifier, comprising: at least one optically amplifying unit having: a forward excitation light source for generating a forward excitation light; a forward excitation light coupler for coupling the forward excitation light with an input optical signal; an optically amplifying medium for amplifying the input optical signal combined with the forward excitation light; a backward excitation light source for generating a backward excitation light; a backward excitation light coupler for coupling the backward excitation light with the amplified optical signal; a first controller for controlling the forward excitation light source to generate the forward excitation light with a power up to a maximum power thereof; and a second controller for controlling the backward excitation light source to generate the backward excitation light in such a way that the backward excitation light compensates gain deficiency as compared with a desired gain of the optical amplifier.
In the optical amplifier, the first controller may control the forward excitation light source to generate the forward excitation light with the maximum power thereof.
In the optical amplifier, the optical amplifying unit may further comprise: a first directional coupler for deriving a first branch signal of the input optical signal; and a second directional coupler for deriving a second branch signal of the amplified optical signal, and the second controller may control the backward excitation light source on the basis of the first branch signal and the second branch signal.
In the optical amplifier, the optical amplifier may comprises a plurality of the optically amplifying unit connected in series.
The optical amplifier may further comprise: an optical output adjuster inserted between two of the optically amplifying units, wherein the optical output adjuster may adjust the level of the optical signal which passes therethrough.
The optical amplifier may further comprise: a transmission path characteristic compensator inserted between two of the optically amplifying units, wherein the transmission path characteristic compensator may compensate dispersion and/or gain of the optical signal which passes therethrough.
According to a second aspect of the present invention, there is provided an optically amplifying method, comprising the steps of: generating a forward excitation light; coupling the forward excitation light with an input optical signal; amplifying the input optical signal combined with the forward excitation light; generating a backward excitation light; coupling the backward excitation light with the amplified optical signal; controlling the forward excitation light to be generated with a power up to a maximum power thereof; and controlling the backward excitation light in such a way that the backward excitation light compensates gain deficiency as compared with a desired gain of the optical amplifier.
In the optically amplifying method, the forward excitation light may be controlled to be generated with the maximum power thereof.
The optically amplifying method may further comprise: deriving a first branch signal of the input optical signal; and deriving a second branch signal of the amplified optical signal, and wherein the backward excitation light source may be controlled on the basis of the first branch signal and the second branch signal.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of the best mode embodiment thereof, as illustrated in the accompanying drawings.